3d world studio seems not to work with unity, can anybody name any good program (not expensive) that i could use to make interiors?
If your speaking about modeling, the cheapest (it's free) program must be "Blender".
Hours of tutorials but very powerful tool.
If you're looking for an easy to use 3D modelling tool, you also might want to check out Cheetah 3d. It's very easy to pick up and should be perfectly fine for creating interiors. It also works very well with Unity. Only works if you're on Mac, though (it's Mac only). It's not free but at $149 very affordable (IMHO). I'd also recommend Modo - but that's $995 so I'd consider that "somewhat expensive".
Blender is pretty good & easy. However i would suggest buying unity3d assets from asset store. It will save a lot time if you are new to modeling.